Here's what I do:
1) Google it (hat name + effect), first result will probably be a page from
www.tf2pricecheck.net. For instance,
http://www.tf2pricecheck.net/Thread-PC-Smoking-L-inspecteur. On the thread for your hat, only pay attention to posts that are within the last month or two (as prices fluctuate a lot).
2) Take a look on backpack.tf/unusuals. This may give you an idea, but again they may be out of date. For instance, yours would be:
http://backpack.tf/unusuals/622/L%27Inspecteur.
3) Now, the important part. Go to TF2 Outpost and search for the hat in question (specify unusual quality). A search for yours would be at:
http://www.tf2outpost.com/search/34121093. Look through similar trades and get a feeling for what they seem to be going for. You can also look at ones with different effects, if they are similar in tier (e.g. steaming price should probably be close to the smoking price, most of the time).
There are other trading sites (as alternatives for step 3), but you should have a pretty good idea overall if you follow these steps. My opinion is that your hat is worth around a bud, maybe a little bit more (e.g. +small number of keys).